It is known that internal combustion engines are equipped with spark plugs and that the spark plugs may be of the type of resistor-incorporated spark plug wherein a spark plug is formed of a refractory insulator containing therein a center bore in which a sparking center electrode and a terminal electrode are inserted and sealed at both the ends of said bore, respectively, and which bore is filled with a resistor made of a resistor composition and sealed between the center electrode and the terminal electrode, the resistor being provided to attain a given resistance value for the purpose of preventing the occurrence of electric wave noise interferring the radio frequency wave upon ignition or spark discharge of the spark plug (see, for example, U.S. Pat. Nos. 4,173,731 and 4,482,475 specification). It is naturally desirable that the resistor should have a long and stable service life under discharge load as much as possible and should not be likely to deteriorate in its effect for prevention or suppression of the electric wave noise.
For the stabilization of the service life characteristics under load of the resistor composition for spark plug, for example, the electric aging method is known in which the resistor composition is electrically stabilized by application of high electric voltage. For this method, there was proposed a technique where a water-soluble carbonaceous substance is added into the resistor composition to prevent such reduction in the resistance value which is inevitably caused by the addition of carbon black (Japanese Patent Publication No. 22505/72). There was also proposed a technique where a resistor composition comprising glass/aggregate/carbon is incorporated with oxides and carbides of metals of groups IVa and Va etc. of Periodic Table (Japanese Patent Publication No. 19721/81), and so on. However, these days, spark discharge energy for spark plugs has been enhanced to meet a required high output of the engines. With known resistor compositions, the carbon present as one of the ingredients of the resistor compositions can be oxidized (burnt out) due to the enhanced current energy of the spark discharge running through the resistor in use, so that the resistor is likely to increase the resistance value during service. When a large amount of an agent for stabilizing the service life under load (such as TiO.sub.2), namely an agent for stabilizing a change in the resistance value of the resistor with time during use (hereinafter abbreviated as stabilizing agent for the resistance) is added to the resistor composition to prevent such a phenomenon, the service life characteristics under load of the resistor can be improved, but reversely the temperature coefficient of the resistance value is deteriorated whereby the electric wave noise-preventing characteristics are deteriorated. On the other hand, Japanese Patent Publication Nos. 51142/81 and 60761/82 describe such self-sealing glassy resistor compositions which consist of glass-metal oxide and carbide (the stabilizing agent for the resistance under load)-carbon-metallic powder or alloy powder. However, these self-sealing glassy resistor compositions do not contain any aggregate or ceramic powder, and hence they show poor stabilization of the resistance value and a low effect for prevention of the electric wave noise, and therefore they cannot provide such resistor composition which is satisfiable sufficiently to cope with the increased spark discharge energy involved by the today's engines of the high output.
Further, Japanese patent application first publications "Kokai" No. 144830/75, No. 68131/74 (corresponding to DT-OS 2245404) and No. 105988/82 describe some examples of such spark plugs in which a resistor is incorporated in an insulator within spark plug to prevent the interference on communication electric wave as invoked at the time of the radio frequency waves being emitted from the high voltage ignition circuit of internal combustion engines.
In the resistor composition according to Japanese patent application first publication "Kokai" No. 144830/75, a ceramic filler (i.e. aggregate) having a relatively large particle size is used, while a glassy aggregate is used in the resistor compositions according to the above-mentioned two Japanese patent application first publication Nos. 68131/74 and 105988/82. In accordance with the invention of the Japanese patent application first application "Kokai" No. 105988/82, a glassy aggregate made of a glass of lithium/calcium boro-silicate base and having a relatively large particle size (all particles passing through 16 mesh) is used and is mixed with carbon black, a glass powder and an electrically insulating ceramic aggregate powder. Such glassy aggregates having the relatively large particle sizes can provide such resistor of te type that the internal structure has been blocked and the passages for electric current are zigzaged and so elongated, whereby the electric wave noise-preventing characteristics of the resistor are improved. In this resistor, however, the passages for the electric current are narrowed so that the density of the discharge current passing through said passages is increased, resulting in that the carbonaceous substances such as carbon present in the conventional resistor composition can be oxidized (burnt out) during service, rendering the resistance value to increase unduly. Moreover, the performances of the known resistor compositions become insufficient, as the spark discharge energy has been enhanced in the recent years to meet the high outputting of engines.
One object of the invention is to provide an improved resistor composition for resistor-incorporated spark plug, which exhibits a sufficiently stabilized service life under load to cope with the increased spark discharge energy involved by high outputting of today's engines.
Another object of the invention is to provide an improved resistor composition for resistor-incorporated spark plug, which exhibits a sufficiently stabilized service life under load as well as an enhanced effect of preventing the electric wave noise to cope with the increased spark discharge energy as produced by the high outputting of today's engines.